The present invention relates to a ceramic silicon nitride based material suitable for machining of nickel- and cobalt-based materials, sometimes designated as heat resistant super alloys (HRSA) with good notch wear, acceptable flank wear and sufficient toughness.
Ceramic materials for cutting tool applications are, thanks to their high hot hardness, suitable for machining work-piece materials of high hardness, high tensile strength at elevated temperatures and low heat-diffusivity, and particularly so for self-hardening materials such as, e.g., some types of nickel- and cobalt-based materials, sometimes designated as heat resistant super alloys (HRSA).
Many silicon nitride based materials for cutting tools are manufactured using aluminum oxide (Al2O3) as a sintering aid. Aluminum and oxygen have the ability to replace silicon and nitrogen respectively in the crystal structure of silicon nitride, thereby creating a so-called sialon ceramic, Si—Al—O—N, sometimes additionally stabilized by a cation Men+, where Me can be chosen from a large number of (rare-earth) metals and lanthanides of suitable ionic radius (r<1.0 Å), such as Y, Yb, Dy, Lu, Li, Ca, Mg, Sc etc.
Many sialon phases have been detected and characterized, see e.g., Izhevskiy et al., “Progress in SiAlON ceramics”, J. Eur. Ceram. Soc. 20, 2275-2295 (2000), but the predominant phases used in cutting tool materials remain α-sialon phase, RxSi12−(m+n)Al(m+n)OnN(16−n) (1.0 less than about m less than about 2.7; n less than about 1.2), where R is one of the aforementioned metals or lanthanides with ionic radius less than about 1.0 Å, and β-sialon: Si6−zAlzOzN8−z were z is greater than zero and less than about 4.2.
During sintering, the raw materials used, usually a mixture of silicon nitride, alumina and AlN or some sialon “polyphase” (or “polytype”), such as 12H, 21R etc., together with an oxide of the metal or lanthanide, form a transitionary melt from which the α- and β-sialon phases, and possibly other phases such as (if Y is used as the metal ion R mentioned above) YAG, melilite, B-phase, 12H etc. crystallize. After sintering, an intergranular phase between the crystalline grains remains. The amount of intergranular phase produced is influenced by the composition of raw materials used, as well as the sintering conditions.
Besides stabilizing the α-sialon phase, the metal ion also functions as a catalyst for the formation of sialon crystals during sintering, and aids the formation of elongated sialon grains, usually in the beta phase, but elongated grains of α-sialon have also been produced. Fang-Fang et al, “Nucleation and Growth of the Elongated α′-SiAlON”, J. Eur. Ceram. Soc. 17(13) 1631-1638 (1997). It is also clear, that the choice of metal ion used affects the properties of the amorphous phase. Fang-Fang X, Shu-Lin W, Nordberg L-O and Ekström T, “Nucleation and Growth of the Elongated α′-SiAlON”, J. Eur. Ceram. Soc. 17(13) 1631-1638 (1997); Sun et al., “Microstructural Design of Silicon Nitride with Improved Fracture Toughness II: Effects of Yttria and Alumina Additives”, J. Am. Ceram. Soc. 81(11) 2831-2840 (1998); Hong et al., “The effect of additives on sintering behavior and strength retention in silicon nitride with RE-disilicate”, J. Eur. Ceram. Soc. 22, 527-534 (2002); Becher et al., “Compositional Effects on the Properties of Si—Al—RE-Based Oxynitride Glasses (RE=La, Nd, Gd, Y or Lu)”, J. Am. Ceram. Soc. 85(4), 897-902 (2002).
The z-value in the β-sialon phase, Si6−zAlzOzN8−z, affects the hardness, toughness, and grain size distribution in the sintered material. Ekström et al., “SiAlON Ceramics”, J. Am. Ceram. Soc. 75(2), 259-276 (1992). The z-value relates to the amount of Al and O dissolved in the Si3N4-lattice. A theoretical z-value can be calculated from the composition of the starting materials. The actual z-value of the beta sialon phase after sintering can be measured by X-ray diffraction analysis. The measured z-values are always somewhat lower than those calculated since the intergranular phase contains more oxygen and alumina than the beta sialon phase.
GB-A-2157282 discloses a range of sialon materials suitable for use in metal cutting tools, with and without α-sialon, refractory additions such as TiN and SiC, with or without crystalline AlN etc., but always containing a “polytype” sialon phase.
U.S. Pat. No. 5,370,716 discloses a ceramic material for use as a cutting tool in the high speed machining of high temperature alloys and cast irons with a composition of β-sialon (Si6−zAlzOzN8−z where 1<z<3) and intergranular phase.
U.S. Pat. No. 5,965,471 discloses a sintered ceramic material for high speed machining of heat resistant alloys comprising sialon grains and 0.2-20 v/o intergranular phase. At least 80 v/o of said sialon phase is β-sialon having a z-value 1-1.5. The ceramic material has a Vickers Hardness HV1 of more than 1530 and it is produced by gas pressure sintering.